The terpenoids constitute the largest family of natural products with over 22,000 individual compounds of this class having been described. The terpenoids play diverse functional roles in plants as hormones, photosynthetic pigments, electron carriers, mediators of polysccharide assembly, and structural components of membranes. Farnesyl pyrophosphate is converted to squalene in the first dedicated step towards sterol biosynthesis. Squalene is then converted to squalene-2,3-epoxide which, in photosynthetic organisms, is converted cycloarterenol.
Squalene monooxidase (EC 1.14.99.7), also referred to as squalene epoxidase, is an oxidoreductase which acts on paired donors with incorporation of molecular oxygen. This enzyme is located at the endoplasmic reticulum, and catalyzes the conversion of squalene to squalene 2,3-epoxide in the pathway to produce sterol. Squalene monooxygenase may be the rate limiting step in sterol biosynthesis. Oxygen, NADPH, FAD, and a cytosolic protein are required for squalene monooxygenase function. Squalene monooxygenase together with lanosterol synthase was formerly known as squalene oxydocyclase.
Whereas vertebrates and fungi synthesize sterols from epoxysqualene through the intermediate lanosterol, plants cyclize epoxysqualene to cycloartenol as the initial sterol. This reaction is catalyzed by cycloartenol synthase (EC 5.4.99.8), also called 2,3-epoxysqualene-cycloartenol cyclase.
Sequences encoding peptides with similarities to cycloartenol synthase and squalene monooxygenase are found in the NCBI database having General Identifier Nos. 5566676, 5468642, 5706248, 5608422, 5055959, 2309779, 2310417, 3107671, 702331, and 4874404.